Human Keratinocytes
Human keratinocytes isolated from stratified squamous epithelia can be readily cultivated in vitro (reviewed in Leigh, et al., 1994). Cultivated keratinocytes replicate readily during early passage and can generate large numbers of cells which exhibit certain features of squamous differentiation in vivo. When cultured normal human keratinocytes are transplanted onto mice, epidermal tissue architecture is regenerated over time in an orderly fashion (Breitkreutz, et al., 1997). The ease of cultivation and transplantation of human keratinocytes coupled with the accessibility of skin for the grafting procedure and subsequent monitoring, make this somatic cell type attractive for therapeutic gene delivery. However, due to the initiation of terminal differentiation, transgene expression in keratinocytes is consistently lost regardless of the gene expression strategy used. Several reports have shown that genetically engineered human keratinocytes can recapitulate full thickness epidermis, thus demonstrating that cells with stem cell-like properties were present in the transplanted population of cells (Choate and Khavari, 1997; Choate, et al., 1996; Gerrard, et al., 1993; Garlick, et al., 1991; Greenhalgh, et al., 1994; Vogel, 1993, Fenjves, 1994).
In Vitro Tissue Culture Assays Utilizing Human Cells Derived from Stratified Squamous Epithelia
In vitro assays using monolayer cultures of adherent cells which maintain the normal in vivo tissue context do not exist for human tissues. Animal models do have the capacity to mimic some of the processes involved in the response of human tissue therapies. However, animal systems lend themselves only to qualitative and subjective scoring of tumor repopulation. Historically, simple in vitro growth assays have used monolayer cultures of rodent or human cell lines on plastic tissue culture dishes. Colony size or cell number are assessed in order to estimate the extent of survival and repopulation of cancer cells following radiation treatment. A major drawback of this approach is that it does not account for any adhesive or paracrine growth factor signals within the tumor cell environment. For this reason, studies on the growth of tumor cells in the absence of normal surrounding tissue may not accurately reflect the in vivo growth characteristics of tumor cells.
For example, human head and neck (H&N) tumors are diagnosed in 43,000 patients in the United States every year and in over 750,000 patients worldwide. Although tumor recurrence near the site of the primary tumor is the predominant cause of treatment failure and death for these patients, little is know about the molecular events contributing to tumor regrowth following treatment. Clinical and radiobiological evidence suggests that tumor proliferation rates may actually increase following wounding due to radiation exposure (Hall, E. J., 1988; Petereit, D. G., et al., 1995). It has been suggested that the wound environment provides potent tumor growth signals (Haddow, A., 1972). For example, extracellular matrix (ECM) glycoproteins present in the wound bed provide and/or sequester potent growth stimuli required for normal tissue regeneration. It is clear from these observations that the tissue context in which a tumor initially develops and/or regrows following failed cancer treatment may have significant impact on tumor growth.
Needed in the art of cell biology is a spontaneously immortalized human keratinocyte cell line with near normal chromosomal complement and a method for using this immortalized cell line in an in vitro tissue assay.